Syd Barrett without question...I am surprised he has barely been mentioned. Piper at the Gates of Dawn is by far the best album of the late '60s psychedelic movement and quite possibly the best album in the bands career. He just had a natural touch to songwriting that both Roger and Dave didnt quite seem to have. Sure, they were fantastic songwriters as well, but even they will admit to it, which is made apparant on the 'Syd Barrett story' documentary. There was just this magical feel to his songs that I can't really describe and I can certainly say that few could ever match.
I agree with you that syd was amazing (RIP). I also think that Piper is the best psychdelic 60s album (followed by Sgt. Peppers and Satanic Majesties, IMHO). I think DSOTM is a better Floyd album though. Aside from Piper, the singles not on that album, such as See Emily Play, Arnold Layne, and Candy and a Currant Bun are outstanding as well. He declined very quickly though, as I was not that impressed with his post-Floyd solo Madcap Laughs album.